Ship propulsion systems can be of various types. In order to reduce fuel consumption and pollutant emissions to the atmosphere, it has been proposed to use a principal heat engine that drives a line shafting with a propeller and a steam turbine that recycles the exhaust gas of the principal engine to drive the line shafting together with the heat engine.
Another type of propulsion system comprises a principal heat engine, e.g. a two-stroke diesel engine, dedicated to driving the line shafting and comprising an auxiliary electrical motor coupled to the line shafting and supplied by a distribution network.
The distribution network is itself supplied by generators driven by auxiliary heat engines.
The exhaust gas pressure of the principal heat engine is used to drive a power turbine, and the heat of the exhaust gas is used in a heat exchanger to drive a steam turbine. These two turbines are associated with a generator that supplies the distribution network.
The electrical power available in the onboard electrical system can be used to power various tasks onboard the ship or for its propulsion.
The systems recovering the heat energy generated during the operation of a heat engine, in this case a principal heat engine driving the line shafting or auxiliary heat engines to produce electricity, are referred to as WHRS (for “Waste Heat Recovery System”).
More particularly, the pressure of the steam admitted into the steam turbine is controlled in order to act on the power consumed or produced.
The propulsion system provides for reversible operation.
According to a first operating mode, the coupled auxiliary electrical motor contributes to the drive of the propeller shaft by taking energy from the distribution network via a converter. This operating mode is generally referred to as PTI (for “Power Take IN”).
The auxiliary motor can also operate in generator mode and deliver electrical energy to the distribution network via a converter. This operating mode is generally referred to as PTO (for “Power Take Off”).
Thus, if the power supplied by the steam turbine increases to a high value such that the steam turbine is more heavily loaded, the auxiliary electrical motor coupled to the line shafting operates in motor mode so as to increase the amount of energy consumed.
On the other hand, if the pressure in the steam turbine decreases, the coupled auxiliary motor operates in generator mode via a converter in order to reinject the power required to supply the distribution network.
Another control technique of the WHRS system is to use a control device which generates a control signal of the steam turbine valve according to the speed and power.